For most situations, this will never matter to you at all. Different types of glass absorb more or less light than other types of glass so there's simply no guarantee as to how much light will actually transmit through the lens. What this means is that F5.6 on one lens might not yield the same level of brightness as F5.6 on another lens. Instead it's a measure of the relative change in light from one aperture to the next. With F stop numbers it's important to know that the F number is not a measure of a specific level of brightness. 5.6, 3.5, those end up in our F stop scale. Since the calculation of the F stop involves the area of a circle, we end up with fractional numbers. That second number, the aperture, is a circle of course, and that's why F stop numbers are so strange. It's derived from a simple ratio, the ratio of the focal length of the lens to the size of the lens' aperture. You may not, though, have ever learned exactly what the F number itself means. Changing the aperture makes for a brighter or darker image and because of the physics of light, changing the aperture alters the amount of depth of field in your image. You probably know that changing the F stop changes the size of the aperture inside the lens which allows more or less light to pass through the lens. And you probably know that when you set the aperture on your camera's lens, you do so using an F stop scale. If you've got enough of an interest in photography that you're watching this video, then you are probably already familiar with the term F stop.